Tuesday, August 16, 2016

Reef Sharks prefer bite-size Meals!

"Although black-tip, white-tip and grey reef sharks have long been thought of as top predators, we found that the chemical structure of the sharks' body tissue actually matched closely with that of large reef fishes such as groupers, snappers and emperors,"."This result tells us that reef sharks and large fishes have a similar diet, but they don't eat each other. So rather than eating big fish, reef sharks are eating like big fish.""We now know that reef sharks are an important link in the food chain, but they are not the last link in the food chain. In most cases, the top predators are tiger sharks, hammerhead sharks, or people,"

Assignment of species into discrete trophic groups is
standard protocol in community ecology and has facilitated
unique insights into ecosystem function and alternative
management scenarios, which are ultimately used to guide
policy decisions.At present, reef sharks are typically
assigned to the apex of food webs, but our results
indicate that this practice misrepresents trophic structure
among high TP species.Hence, we advocate a reassignment
of reef sharks to an alternative trophic group (such as
high-level mesopredators) that better reflects trophic similarities
between reef sharks and large predatory fishes. This
change is expected to refine our understanding of how reef
communities function, and ultimately, improve management
of reef sharks.If indeed reef sharks are high-level mesopredators, who
then are the apex predators on coral reefs? Given their
superior size and ability to eat reef sharks, we hypothesise
that the role of apex predator is fulfilled by large, roving
sharks such as G. cuvier, C. obscurus, C. albimarginatus, N.
acutidens and S. mokarran. And surely Bull Sharks - or not?Although large roving sharks are
seldom seen during visual surveys of coral reefs and thus are typically considered rare, their
actual abundances may be much higher than currently
believed, since they accounted for approx. 9 % of all sharks captured
by long-lining at our study sites (excludes N. ferrugineus) and they comprise a high proportion of
sightings by baited remote underwater videos on the GBR. Therefore, it is plausible that large
roving sharks are present in sufficient numbers to potentially
exert top-down control of reef sharks and other high-level
mesopredators on coral reefs.Removal of apex predators such as wolves, lions and
dingoes can invoke trophic cascades due to release of
numerous prey species and subsequent flow-on effects to
lower trophic levels. However, trophic cascades induced solely by removal of
reef sharks are rare, subtle and/or equivocal, implying
that reef sharks have relatively weak effects on community
structure and function.

A potential explanation is that
functional redundancy exists among large piscivores, such
that equivalent species compensate for any loss of reef
sharks and thus buffer potential trophic cascades. This
hypothesis is supported by our results, which indicate that
(1) reef sharks and large predatory fishes are functionally
similar (based on equivalent mean TPs and overlapping
isotopic niches, and (2) these two groups
of predators are dietary generalists and potentially consume prey in
proportion to availability, thereby compensating
for loss of species-level interactions. It is also
noteworthy that large predatory reef fishes are highly
diverse (more than 20 species on the GBR) and probably encompass
a broader range of trophic niches than those of the four
species considered here. In view of these results, we contend
that functional redundancy exists among large piscivores
and is sufficiently high on the GBR to stabilize
community structure despite moderate to high fishing
pressure and depletion of reef sharks in some areas.

We conclude that large conspicuous predators, be they elasmobranchs or any other taxon, should not axiomatically be regarded as apex predators without thorough analysis of their diet. In the case of reef sharks, our dietary analyses suggest they should be reassigned to an alternative trophic group such as high-level mesopredators.

There you have it.

So let's please stop proffering the same old tired nonsense - it is false in its generalization and as such, it is nothing but bad conservation.